The air traffic controller, also called ATC, the acronym standing for “Air Traffic Control”, generally gives the crew of an aircraft a start-of-approach time so that they apply the approach procedure at the opportune moment. It may happen that the traffic around an airport is saturated, the traffic and/or the congestion of the runways not making it possible to satisfy a landing at the initially indicated time.
Certain situations then induce the air traffic controller to request certain aircraft to fly a holding circuit for a duration deduced from the time of final approach to be performed. The approach time generally being denoted SAT, standing for “scheduled approach time”, this resulting in a holding circuit exit time. To satisfy the flight conditions of a holding circuit in complete safety, functions, called “HOLD functions”, are provided for by the “Arinc 424” standard in certain terminal procedures. They make it possible notably to manage aircraft holding for a predetermined duration in a holding circuit. Conventional flight management means of the aircraft such as an FMS, the acronym standing for “Flight Management System”, make it possible within this framework to manage a holding circuit exit time so as to commence the landing procedure.
Currently, the air traffic controller can request an aircraft to stop its HOLD procedure so as to exit therefrom. In this case, it is up to the pilot to extrapolate the holding circuit exit instruction as a function of the position of the aircraft in the holding circuit at the present instant so as to calculate the optimal trajectory of the aircraft in order to be as close as possible to the exit point at the indicated time.
Generally, the holding circuit, bearing the same name as the HOLD function, has the form of a racecourse comprising two circular arcs, which can substantially be two half-circles, and two branches forming two parallel straight lines linking the ends of each half-circle.
Hereinafter, one of the two parallel straight lines of a holding circuit which links one circular arc to another will be called a branch of a HOLD.
The holding circuit furthermore comprises an exit point and an entry point which can be substantially the same or be opposite in the holding circuit as the case may be. Generally they are situated at the ends of one of the branches of the holding circuit, therefore just before a turn. These points can be tagged with respect to a beacon in proximity to the airport and their coordinates are known in latitude and longitude.
The branches forming straight lines of the holding circuit, in main airports, are normally traversed over a duration of 1 or 1.5 minutes. Certain holding circuits have straight segments defined by a distance that is easily measurable in flight with a navigation instrument. This convention allows the crew to express the distances in the form of temporal constraints. When they fly the holding circuit at constant speed the crew therefore obtains simple temporal tags to attain the exit point of the holding circuit.
These two branches are therefore generally flown at constant speed in a regime making it possible to optimize consumption. The branch on which the aeroplane arrives is called the “inbound leg” and the other branch parallel to the latter is called the “outbound leg”
Numerous patents describe aircraft entry or exit procedures when said aircraft flies a holding circuit.
Patent application WO 2004/059252 describes notably the entry and exit pattern of an aircraft when the latter enters and leaves a holding circuit. But an aircraft's holding circuit exit procedure is not automated, the air traffic controller gives the aircraft crew an indication such as the exit time such that the aircraft attains the exit point of the circuit as quickly as possible. In this case the aircraft crew calculates approximately the trajectory of the last loop so as to be at the exit point of the holding circuit at the time indicated by the air traffic controller.
Currently, no automatic predictability exists making it possible to optimize the form of the holding circuit in the last loops so that the aircraft is at its exit point at the known exit time. Currently, depending on the knowledge of the situation of the aircraft, the air traffic controller must estimate the moment at which the aircraft must decide to leave the HOLD, assuming that it will be reinserted into the traffic a few minutes later. The air traffic controller must also give or confirm the exit HOLD instruction to the crew in the last loop of the holding circuit.
It is possible today to input a time constraint on the exit point of a holding circuit, that is to say a time at which the aircraft must be at its exit point. This allows the air traffic controllers to give the aircraft the exact time at which it must reinsert itself into the traffic in advance. On the other hand, when the aircraft enters the holding circuit, the flight plan is not capable of adapting to comply with the constraint, since the size of the holding circuit, notably the portions of branches of the racecourse, are fixed.
Currently, the time constraint given to the crew regarding exit from holding circuits only allows the function to give the crew a prediction to tell them whether “yes” or “no”, the constraint will be complied with. The pilot is not aided or assisted in optimizing his trajectory so as to be at the exit point of the holding circuit at the end of the time constraint.
Notably, in certain aircraft, the FMS predicts only that the aircraft will or will not finish the loop of the holding circuit commenced (with the initial fixed size), by specifying whether it must leave the holding circuit and embark on the rest of the flight plan or whether it must continue one more loop of the holding circuit.
It is common for aircraft to be placed on hold at the end of descent or the start of approach on a holding circuit, doing so in order to reinsert themselves appropriately into the final approach traffic.
The drawback of the existing solutions is that in no case can the flight plan accommodate the time constraint given by the air traffic controllers. Insertion into the traffic then remains approximate as regards compliance with the temporary constraint by the crew.
Pilots can use an empirical formula to calculate a postponement time to satisfy the time constraint. The latter calculation remains complex and precision is not guaranteed according to the direction of the wind and of the last loop of the holding circuit which can sometimes be flown in “heading hold” mode by hand to return to the exit of the holding circuit at the right moment. Generally the workload takes up the entire resource of the pilot.